Among the conventionally-known schemes for supplying fuel from a fuel tank to an internal combustion engine are a first fuel supply scheme in which fuel is supplied from the fuel tank to a carburetor, and a second fuel supply scheme in which fuel is supplied from the fuel tank to an injector. The second fuel supply scheme has been popularly used in recent years because it allows a fuel injection amount of the injector to be controlled finely by means of an electronically-controlled fuel injection device. An example of such a fuel supply device for an internal combustion device is disclosed in Japanese Patent Application Laid-Open Publication No. SHO-64-73165 (hereinafter referred to as “the relevant patent literature”).
The fuel supply device disclosed in the relevant patent literature comprises a combination of a transfer pump and a diaphragm pump, and it is employed particularly in a utility engine. The transfer pump transfers fuel from within the fuel tank to the diaphragm pump, and the diaphragm pump supplies the fuel, transferred by the transfer pump, to the injector by use of pressure variation within a crankcase.
Many of the commonly-known small-size utility engines are of a type which includes a manual recoil starter and a power generator (alternator) without including a battery. The transfer pump requires a drive source, such as an electric motor. A power generator of a relatively great power generating capability is required to secure activating electric power for the transfer-pump driving motor. Further, a great starting operation force is required to activate such a power generator of a relatively great power generating capability, and thus, the prior art fuel supply device still has room for improvement.
If the transfer pump requiring a drive source is dispensed with or omitted, another fuel supply scheme might be employed, in which the fuel pump is disposed above the diaphragm pump so that fuel can be supplied from the fuel tank to the diaphragm pump by gravitational force.
However, delivery pressure with which fuel is discharged or delivered by an ordinary diaphragm pump is relatively small. If the fuel delivery pressure is small, differential pressure before and behind an orifice provided at the distal end of the injector would become small. Such small differential pressure is disadvantageous if it is desired to increase accuracy of a flow rate of fuel to be supplied from the injector to a combustion chamber. Besides, in the case where the fuel delivery pressure is small, arrangements must be made for preventing so-called “vapor lock” even when the fuel is low in temperature. Note that the “vapor lock” is a phenomenon where air bubbles are produced by liquid fuel being vaporized by ambient heat and thus a fuel system is undesirably closed with the thus-produced air bubbles.
Further, in order to lubricate sliding portions accommodated in the crankcase, ordinary internal combustion engines are constructed to produce oil mist by stirring up and scattering lubricant oil within the crankcase. In case the oil mist enters the diaphragm pump, it can undesirably adhere to components parts within the diaphragm pump.